1. Field of the Invention
The present invention relates to a small cylindrical type radial-gap rotary electric machine having improved mounting accuracy of a rotor shaft and heat dissipating efficiency of a bearing and, more particularly, to a rotary electric machine with an armature structure in which a salient pole is formed separately from a stator ring, each salient pole is divided, and a bobbin is used for each salient pole with concentrated winding.
2. Description of the Prior Arts
Generally, an inner rotor type brushless DC motor has a stator portion arranged on the outside thereof, and attached directly on a metallic case or the like, so that heat generated in the motor, especially in the stator portion, is easily dissipated outward through the metallic surface of the stator or is transferred to the case or the like. Thus a good cooling efficiency is provided. The applicant of the present application proposed in Japanese Patent Application No. 178687/1997 an electric machine wherein resin is injected in between a case of the motor and a stator portion to form them into one piece and the resin is also used to form one of the flanges. This proposal brings such advantages that filling of the resin allows the stator to be secured to the case, and flanges for supporting bearings are formed into one piece by the resin filled inside the case to provide improved accuracy of fabrication and increased durability.
However, there was a problem with the foregoing rotary electric machine as above mentioned that flanges for supporting the rotary shaft are formed of resin and, therefore, have insufficient mechanical strength compared with metallic flanges.
For example, in the case where a printing head is driven with a belt looped over a drive shaft of a motor of a printer or the like, increasing belt tension to improve the positioning accuracy of the printing head causes the side pressure applied to the drive shaft to become larger, thereby sometimes resulting in looseness between the shaft and these resin-made flanges due to poor strength for supporting the drive shafts.
Furthermore, resin has lower thermal conductivity compared with metal and thus has such a disadvantage that heat generated in the resin is less prone to being transferred to the outside. That is, a problem arises when flanges are formed of resin and bearings are covered with resin, because heat generated in the bearings is liable to be captured, thus resulting in an increase in the bearing temperature. Furthermore, increasing the thickness of the resin to raise the mechanical strength of the flanges causes the amount of resin used to increase, and accordingly causes the size, the weight and the cost of the motor to increase, and causes the heat resistance and temperature to increase as well.
Accordingly, in the foregoing example, heat is not transferred to the case surface or the like via the flanges whereby temperature at the bearings supported by the flanges increase extraordinarily during operation. In general, it is known that the life of bearings reduces significantly as the temperature increases during operation. Considering the life of a rotary electric machine, the heat dissipating efficiency of a bearing is as important as the heat dissipating efficiency of the stators.
As mentioned above, in the inner rotor type brushless DC motor that is said to have good heat dissipating efficiency and high durability, it is very important to improve heat dissipating efficiency at the bearing portion in increasing the life of a whole motor. In this sense, the proposal made in Japanese Patent Application No. 178687/1997 has an improved aspect in which flange portions are formed of a resin having low thermal conductivity or high heat resistance.